Most wells, such as oil and/or gas wells, require fluid to be injected for a variety of requirements. These may include but are not limited to:                Chemical Injection—where chemicals for the mitigation of phenomena such as scaling, wax build up, salt built up etc. are addressed by the injection of speciality chemicals which are formulated to address such issues. These applications tend to be performed at very low flow rates which are a very small fraction of the flow rates of the actual produced reservoir well fluids.        Water De-Salting Injection—where water of either a pure or derived composition is injected to assist in the flushing away of salt deposits in the oil producing region in oil or gas formations. These applications are generally performed at moderate rates of flow which are a small fraction of the flow rates of the actual produced reservoir well fluids.        Diluent Injection—where a fluid of a special composition is injected with the specific purpose of acting as a solvent to reduce viscosity and density of reservoir fluids in order to allow them to be more pumpable to improve or allow production to surface by methods such as a down hole mechanical pump, a down hole electric submersible pump (ESP), gas lift or other such methods of artificial lift. These applications tend to be performed at moderate to high flow rates which are a greater fraction of the flow rates of the actual produced reservoir well fluids.        Direct Water injection—where water recovered from another well is injected in order to replenish reservoir pressures and volumes in order to assist in the production of other wells. This is generally performed at very high rates of flow comparable to the production flow rates that may occur form other producing wells.        
Injection devices or valves are typically used to facilitate injection into a wellbore. Different types of injection device may be used depending on the nature of the injection, such as chemical type, flow rates etc. Some valves may operate to seek to provide a fixed pressure differential between inlet and outlet, for example by use of a power spring acting against a valve member. Further, some valves, such as disclosed in WO 2014/037584, the disclosure of which is incorporated herein by reference, seek to maintain an injection line in positive pressure, to avoid issues associated with negative or reduced pressures being present, for example caused by a U-tube effect, which might occur where injection fluids cascade through the injection line and injection valve to seek a hydrostatic equilibrium with the wellbore at the point of injection. This is particularly the case where operation of an injection valve is dependent on tracking the outlet pressure.
In some known valve designs, a flow path is selectively opened and closed by a valve member engaging and disengaging a valve seat. In some instances relative movement between the valve member and seat is caused by use of a sensor piston arrangement which moves in accordance with a differential between inlet pressure and a reference pressure. The sensor piston arrangement typically includes a sealing assembly exposed, in opposing directions, to the inlet and reference pressure s respectively, to permit pressure based control over the movement of the sensor piston. However, in many instances it is important for the sealing assembly of the sensor piston to define a larger, and in some cases a significantly larger diameter or area than the area of the valve seat. This firstly ensures that the pressure force applied over the area of the valve seat can be overcome by the sensor piston, and secondly assists to minimise hysteresis of operation as the valve member and valve seat are moved relative to each other from closed to opening and intermediate flow regulating positions.
However, in some instances, for example in downhole locations requiring very high injection flow rates, space is restricted to accommodate both large flow areas and a large ratio of areas between the sensor piston and the valve seat.